Two basic types of ground fault sensing systems for sensing ground fault currents in AC-to-AC converter circuits have heretofore been developed. The first such prior art system typically employs an input or output ground fault transformer which is somewhat complicated and thus inherently adds significant cost to the overall controller. The second type of prior art ground fault detector conventionally employs a current sensing element such as a Hall effect device in the DC link between the controller bridge rectifier and the DC-to-AC converter. DC link current feedback sensing configurations, however, suffer from either an inability to detect all possible ground current conditions or from larger than desired current trip levels. It would accordingly be of great benefit to have available a ground fault current detector which is capable of effectively detecting ground fault conditions in alternating-polarity control systems at low current trip levels while making use of inexpensive circuit components.
It is therefore an object of the present invention to provide a ground fault detector for use with alternating polarity power supply systems.
It is a further object of the present invention to provide a ground fault detector for AC-to-AC converters, wherein the ground fault detector makes use of inexpensive circuit components which detect ground faults in the AC-to-AC converter circuitry at low ground fault current trip levels.
It is yet another object of the present invention to provide a ground fault detector for AC-to-AC converters, wherein the ground fault current detector relies upon current feedback from the converter DC link to supply an indication of ground fault conditions at current trip levels low enough to enable controller shut-down to occur before the AC-to-AC converter power transistors are damaged.
These and other objects of the present invention are achieved in a ground fault detector employing a current feedback loop which makes use of a differential current sensing configuration to in essence increase the gain of the current feedback signal, thereby lowering the detector trip level when ground fault currents are present. One embodiment of the present invention designed for AC-to-AC converters positions the differential sensing configuration in the DC link of the AC-to-AC converter circuitry.